Our tank does not filter really small stuff like bacteria. The water comes from a rain-water fed tributary of a stream, 25 kilometers up into the mountains. The source is protected from the odd, mad, pig hunter that may travel those inhospitable areas.

I know Legionnaires disease can be caught by inhaling steam or mist of affected water, and also another nasty disease that comes from inhaling fish tank water. I have never heard of either being found in house water tanks.

Every now and then we have the tank water tested and it has always been pristine.